Tanya Degurechaff is a cute, adorable and pretty-good looking little girl. But deep inside, she is highly power-hungry, amoral, ruthless, sadistic and hot-tempered. Especially when she is in battle against her enemies, she shows utter ruthlessness and bloodlust just to accomplish her missions. Tanya also strongly disapproves of failure and disobedience from her subordinates. She is loyal to her own job and always wants to finish it as soon as possible. Tanya will never let anyone discriminate her appearance because of how fast she goes through the ranks in the military.
Originally a salaryman who died in an accident in 2013, he was reborn as a girl living in an alternate universe version of Europe during World War I. At just nine years of age, Tanya enters the Empire's Mages Division and is promoted to Second Lieutenant. Her talents, efficiency, and ruthlessness makes her an asset for the Empire and a threat on the battlefield.
Not much is known about the protagonist in his original life, including his name. He was a Japanese salaryman who gained a reputation for having a ruthlessly utilitarian and career-minded personality. He died in a subway after being pushed in front of a train by a man he fired, after which he meets God. As an atheist, he denies God's existence and rather dubs him Being X. The two get into an argument about faith and modern humans which ends up with God punishing the protagonist by having him reborn in a world that was in dire straits, had less technological development and overall was less grounded in science. The opposite of these three points were used by the protagonist's argument to explain why modern men no longer need God.
The protagonist is reborn in an alternate reality of 1913, as an orphan girl named Tanya. Spending the first years of her life in a nunnery, her outlook for a good life initially looked slim. Worse, she's living in the Empire (equivalent of Imperial Germany), one of the nations that is fighting in World War I (or their alternate equivalent of it). Luckily to her, Tanya has exceptional magic potential, a rare talent sought after in the Empire. After she's discovered to have magical abilities, people quickly expected her to be conscripted into the Empire's military once she reaches an appropriate age.
Tanya however thinks ahead of this, and she volunteers to join the military at the age of 8. Her reasoning was that the Army was the only institution of the Empire where women are treated equal to men. She intends to rise in ranks and then live a comfortable life in the back lines away from danger, both for her sake and in order to spite "Being X." Yet even though Tanya is successful at impressing the leadership it backfires as her talents make her a necessary asset on the front lines. She's heartless, selfish, amoral and terrifyingly ruthless. Yet at the time of the bloodiest war of this world, she might be described as the "ideal soldier." On the outset Tanya is viewed by her squad mates as either a patriotic hero or a bloodthirsty monster.